console.log("offsetHeight", document.querySelector("footer").offsetHeight);
console.log("clientHeight", document.querySelector("footer").clientHeight);
console.log(
  "computedHeight",
  window.getComputedStyle(document.querySelector("footer"))
);
console.log(
  "差值",
  window.innerHeight -
    document.querySelector("footer").getBoundingClientRect().top
);
